During the process of Stretch Blow Molding (SBM) of PET preforms to bottles (in a two-stage process) the reduction in consumption of energy in heating the preforms prior to blowing plays a very important role in the productivity of PET bottles. With the fast cycle blow molding machines, it is critical to have short residence times and shorter equilibration times for the heat-up of Preforms before the actual blowing. The temperature differential between the outer and inner walls of the perform needs to be minimum for ensuring proper blown bottle characteristics.
U.S. Pat. Nos. 2,641,592, 3,028,366, 3,732,182, 3,795,639 3,842,043, 3,907,754, 3,962,189, 3,965,071, 4,010,145 and 4,082,724 all deal with processes for preparing linear polyesters utilizing variety of catalysts comprising Germanium, Antimony, Titanium, Manganese, Cobalt etc. and variety of phosphorous containing compounds as heat stabilizers. However, none of these patents deal with either special additives or the role of catalysts in the heat absorption characteristics of the polymer.
U.S. Pat. Nos. 5,484,837, 5,106,905, 4,879,335, 4,379,871, 4,208,318, 4,116,924, 4,001,171, 3,905,937 & 3,007,891 and JP 49-87792 deal with carbon black addition to PET to produce a black master batch concentrate for colouring polyesters and not related to their reheat characteristics.
U.S. Statutory Invention Registration H2018 H discloses a process wherein fast heat up additives controls the blow temperatures of multilayered preform or container of PET/PEN. Additives like carbon black, graphite, diamond dust and various colorants were tried to bring the two polymeric layers to the respective blow temperatures at almost the same time. In this published work, though the additives are functioning as heating aids, their main purpose is to bring the different layers in a multilayer preform to the same blowing temperature.
U.S. Pat. No. 4,408,004 discloses the use of carbon black (particle size 10-500 nm) in PET in a range less than 10 ppm for producing high clarity, low haze polyester with improved heat up rates. To improve the clarity, phosphates have been used as heat stabilizers along with Cobalt based compounds as bluing agents.
In U.S. Pat. No. 6,022,920 the additives used are black iron oxides Fe3O4 in the sizes between 0.1 and 10 microns.
U.S. Pat. No. 6,503,586 relates to a polyester composition containing inorganic black particles which are copper chromite spinels as I.R. absorbing materials having average particle size of 0.9 and 1.2 micron. These pigments were added at a level of 10-50 ppm to get colourless bottles whose brightness and yellowness is measured. Reheat additives at this high level is likely to cause colour and haze problem in the bottles
U.S. Pat. No. 6,022,920 discloses a process for making PET with improved reheat rates by adding about 5 to 50 ppm black iron oxide particles. In this process the iron particles are added at higher levels viz., 50 ppm to get about 15% less reheat time affecting the L* value and colour of the bottle.
U.S. Pat. No. 6,197,851 presents an invention relating to PET polymer containing one of a series of organic/metallo organic I.R. heat absorbing compounds which includes phthalocyanines, substituted indanthrones and anthrquinones etc. Invariably in such processes controlling the color and haze is difficult.
U.S. Pat. Nos. 5,419,936 and 5,529,744 describe a process of using reducing agents for incorporating metal particles like Antimony in the polyester for reducing the reheat time. If the reduction is not controlled, there is a possibility of the total Sb2O3 getting reduced to Sb, which can result in more graying and haziness in the sheet and bottle apart from reducing the catalytic activity.
The combined teachings of the U.S. Pat. Nos. 6,022,920, 5,419,936 and 5,529,744 disclose methods of obtaining enhanced heating rates. U.S. Pat. No. 6,022,920 makes use of 5 to 50 ppm of black iron oxide particles as additives and U.S. Pat. Nos. 5,419,936 & 5,529,744 deal with in-situ reduction of Sb2O3 to metallic particles like antimony by using reducing agents.
U.S. patent application Publication 2003/0018115 deals with reheat aid additives like FeP, Fe2P, Fe3P, FeSi or the blends of these, which improve the heat-up rates without substantially affecting the colour of the product.